Currently, there exist essentially three different approaches for realizing such a safety system for table and panel sizing circular saws, which are intended to prevent a user from coming into contact with the rotating saw blade or from receiving a severe cut injury as a result of such contact.
The safety system made by an American manufacturer and marketed under the trade name Saw Stop Inc. is an emergency braking system that enables braking of the machine tool on the basis of a direct engagement of a braking actuation mechanism with the saw blade as soon as a correspondingly designed sensor has detected a dangerous situation. Here, through the use of a hot-wire trigger, a rotatably situated aluminum block is pushed into the toothing of the running saw blade using a pre-tensioned spring, said block becoming wedged therein and in this way absorbing the rotational energy of all geometries of the machine tool that are in rotation during the sawing process. As a side effect, this one-sided application of force to the blade is used to sink the saw blade into the saw table through a specially constructed suspension situated on the saw table. Using this system, it is possible to avert severe bodily damage to the operator of the machine tool who triggers the safety mechanism. A disadvantage here is the direct contact with the processing tool, i.e. the blade geometry of the saw blade, because the breaking off of parts of the toothing poses an additional potential risk to the user. In addition, in order to restore the readiness of the safety system, it is necessary to exchange the braking unit, including the saw blade, with a replacement unit ready for use, which the user has to have in reserve in order to continue to work after a braking process using the safety function has taken place. This is connected with significant costs and a corresponding time outlay for acquisition and installation. In addition, it can be assumed that all components involved in the braking process, i.e. all rotating geometries of the machine tool, are exposed to a high degree of stress during the deceleration phase. Neither the manufacturer nor associated publications about this system supply data regarding the fatigue strength of this apparatus.
Another approach uses a safety system solely to lower the saw blade into the saw table without carrying out a braking process of the saw blade. Using a pyrotechnic priming charge, here the saw blade, including the main shaft and its bearing, are removed from the danger zone, so that serious injury to the user can be prevented. A disadvantage of this type of safety system is the necessity of the movement of relatively large masses, in the form of the systems that are to be lowered, under strict time requirements in the range of milliseconds. In addition, the pyrotechnic primers required for this, which turn out to be indispensable for this safety actuator system, result in an expensive partial reversibility, placing a limit in terms of time and also in terms of organization on immediate further processing using the machine tool with a safety system ready for use. Furthermore, due to limited constructive space and a very specific procedure of use, this safety system is suitable only for larger stationary devices, such as table circular saws, which permits such a construction in their interior. This system is not suitable for use with smaller, manually operated devices, such as miter saws and miter box saws or panel saws.
A publication of a publicly promoted project called “Cut-Stop” (VDI/VDE/IT), concerning a safety system for panel sizing saws, of the Institute for Machine Tools of the University of Stuttgart describes an approach that uses a special form of a disk brake installation, namely a self-amplifying wedge brake, to bring the main shaft of the machine tool and thus the saw blade to a standstill. Here, using a pyrotechnic primer, a wedge is accelerated and is subsequently pushed between a stationary wedge guide, in the form of a modified brake saddle, and the rotating brake disc. For the selection and combination of a specific wedge angle α and brake lining values μ, the system works in a self-locking fashion, so that with this design the temporal demands on the braking process can be fulfilled as a function of the mass inertia that is to be decelerated. The disadvantage of this safety system, however, is that here as well, as is also described in the named publication, following the triggering of the safety actuator mechanism it is necessary to exchange the entire braking unit due to the wedge jammed in the friction pair. Here, a time outlay of approximately 10-12 minutes is required in order to completely restore system readiness. Thus, here as well there is a partial limitation of prompt system reversibility.
From DE 195 36 995 A1, a safety brake for elevators is discussed that has a device that, when a maximum speed of the transport device has been exceeded, brakes the drive with a deceleration that is a function of the speed, and also holds it there if warranted. The safety brake from DE 195 36 995 A1 acts immediately, as a function of the rotational speed, on the traction sheave of the cable-operated conveyor installation, and is able to limit the rotational speed thereof. The safety brake, fashioned as a centrifugal brake, additionally has a device for amplifying the braking force as a function of the conveying speed.